In electronic imaging displays, pull down menus, dialog boxes, and other interactive controls typically obscure underlying information. For example, in a personal computer operating system using software controlled display windows, a pull-down menu or "pop-up" dialog box is typically "opaque", completely blocking any underlying information. However, in many cases, it is useful to be able to see underlying information. In particular, if a selection on a menu or dialog box changes what is being displayed in the underlying image, it is useful to see that change before closing the menu or dialog box. For example, in many interactive control situations such as process control, instrument control, and cockpit displays, there may be a need to see the result of a selection before committing to a particular selection. There is a need for superimposed or foreground windows for which some of the foreground information is transparent or translucent, permitting some useful visibility of any underlying information.
A translucent television video image has been implemented as an overlay on a data processing video image. For example, see U.S. Pat. No. 5,265,202 (Krueger et al). In Krueger et al, within a television video window on a data processing display, selected lines of the video image are omitted and the corresponding data processing lines are visible. Within the television video window, both the video and data processing images are partially visible. In Krueger et al, the television image is written into a shared display memory synchronized with video framing and retrieved from memory synchronized with the display screen. Computer generated menus have been implemented as an overlay on television displays. For example, see U.S. Pat. No. 5,604,544 (Bertram). In Bertram, individual menu items are opaque, with a television image visible between the individual menu items. For instrument and control displays, a menu may have relatively small text or icons that need to be read by a human operator. Generally, detailed information may be more readable on opaque menu buttons, as in Bertram, than for overlay images where the entire overlay image is translucent as in Krueger et al.
Both Bertram and Krueger et al require compatibility with television video timing. In each, the ultimate result is that a computer generated image is being merged with a television image. In a computer or instrument, where both images are computer generated or where both images are in digital form in display memory, there are opportunities for less complex and lower cost implementations since display timing is of less concern. Instruments have timing concerns, but instrument timing concerns are dominated by timing of events being measured or controlled, not the timing of the particular display technology. It is possible, of course, in a computer or instrument to implement translucent or transparent windows entirely in software. However, in many instrument systems and control systems, a pure software system is too slow to process real-time events that must be displayed. There is a need for systems with software control over static or infrequently changing parts of a display and hardware control over rapidly changing parts of the display, with at least partial visibility of the rapidly changing parts at all times.